This application claims the priority of German patent document 197 17 998.3, filed Apr. 28, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for the operation of a motor vehicle air conditioner which contains a refrigerant circulating system having a compressor and an evaporator. In this process, the compressor is deactivated to protect against being inadequately filled with refrigerant, due to the refrigerant pressure on the high-pressure side of the refrigerant circulating system falling below a definable limit.
In a process of this type (which is used in motor vehicles produced by the applicant), the protective deactivation of the compressor is accomplished by automatically deactivating the compressor when the refrigerant pressure falls below a definable deactivation threshold value of, for example, 1.75 bar. Here, the reactivation of the compressor is only permitted when the refrigerant pressure exceeds an activation threshold value of, for example, 2.25 bar (which is higher than the 1.75 bar value)
In parallel thereto, the compressor is protected from icing-up by automatically deactivating the compressor when the evaporator temperature falls below a definable deactivation threshold value of, for example, 1.degree. C. Here, as before, the compressor can only be reactivated when the evaporator temperature exceeds a turn on threshold value of, for example, 3.degree. C. (which is higher than the 1.degree. C. value). Together, the two protective deactivation functions prevent damage to the air conditioner (particularly the compressor) when quantity of refrigerant in the refrigerant circulating system is too low or the evaporator becomes iced up.
A common control method for the compressor is the so-called reheat operation. This method entails cooling the air flow blown into the vehicle by the evaporator and subsequently heating the blown air with the heater of the air conditioner. This method is particularly expedient at low outside temperatures and in humid weather when seeking to prevent the vehicle's windows from fogging-up. The windows of the vehicle have an increased tendency to fog up when the air conditioner is restarted after parking for a short time. This is because, once the air conditioner is switched off (that is, when the vehicle is parked), the evaporator normally housed in an air-conditioning compartment will be moist on its surface. Furthermore, the air situated in the air-conditioning compartment will be heated up by the warmer surroundings of the air-conditioning compartment (e.g., the engine compartment) and therefore absorbs water vapor from the evaporator surface. If the air conditioner is then restarted shortly thereafter, this moistened air will enter the vehicle interior, whereby under corresponding conditions, the water vapor absorbed by the air conditioner condenses on the windows.
German Patent Document DE 195 12 356 Cl and the literature cited therein, discloses a process for motor vehicle air conditioners (within the scope of reheating operations) that seeks to remedy this tendency of a vehicle's windows to fog up by, for example, delaying a fan activation with respect to the activation of the compressor when the air conditioner is restarted. In this process of deactivating the compressor to protect against the vehicle's windows icing-up, deactivation of the compressor will (as a rule), however, no longer be activated at outside temperatures below approximately 0.degree. C. As a result, it is no longer possible to counteract the tendency of the windows to fog-up.
An additional disadvantage of this process is that when the protective compressor is deactivated, due to being inadequately filled with refrigerant, activation of the compressor does not take place in additional special operating conditions (even though the windows have a tendency to fog up). The reason for this is, since the refrigerant pressure also depends on the temperature conditions, a refrigerant pressure which is equal to or slightly higher than the pertaining deactivation or activation value (in the event of a lower outside temperature of, for example, approximately -2.degree. C.), still corresponds to a normal refrigerant quantity. On the other hand, at higher outside temperatures of, for example, 0.degree. C. of an insufficient refrigerant quantity, the pertaining deactivation or activation value corresponds to the compressor being inadequately filled with refrigerant. Therefore, when the vehicle is parked at an outside temperature that is slightly above 0.degree. C. (during which time the air conditioner with an activated compressor operating to the cool the vehicle interior), in the event of a subsequent new start, the compressor is not switched on if the outside temperature has fallen in the interim, even if the windows have begun to fog up. This phenomenon is due to the compressor being deactivated to protect against the effects of inadequately filling the compressor with refrigerant.
The object of the present invention is to provide a process whereby, despite implementation of the protective shut-off function when the compressor is inadequately filled with refrigerant, the tendency of the windows to fog up can be counteracted in a comparatively reliable manner.
This and other objects and advantages of the invention are achieved by the process according to the invention, in which the compressor is deactivated to protect against the compressor being inadequately filled with refrigerant, due to the refrigerant pressure (p.sub.k) on the high-pressure side of the refrigerant circulating system falling below a definable refrigerant pressure switch-off threshold value (p.sub.k1).
In this process according to the present invention, the protective deactivation (when the compressor is inadequately filled with refrigerant) is suppressed in the special case where the engine coolant temperature is below a definable coolant deactivation threshold value and the evaporator temperature is above a definable first evaporator temperature deactivation threshold value. As a result of this measure, compressor activation is ensured when the vehicle is parked at a higher outside temperature of, for example, slightly above 0.degree. C., and where the compressor was operating during a drop in the outside temperature (prior to the interim that the vehicle is restarted). In this scenario, the protective deactivation function (when the compressor is inadequately filled with refrigerant) is suppressed as long as, on the one hand, the evaporator temperature is still above the first evaporator deactivation threshold value (which may be set, for example, at approximately 2.degree. C.) and, on the other hand, the engine coolant temperature has not yet reached its defined deactivation threshold value. The automatic deactivation of the compressor for preventing the tendency of the windows to fog up (which can be triggered after any of the conventional approaches) is therefore undisturbed in this operating situation due to the protective deactivation of the compressor when it is inadequately filled with refrigerant.
In contrast, if the evaporator temperature falls under the first evaporator temperature deactivation threshold value, the deactivation of the compressor can be triggered so that damage due to ice on the evaporator is prevented. A compressor deactivation can also (for example) take place because the protective deactivation when the compressor is inadequately filled with refrigerant is permitted. This is because the engine coolant temperature has reached or exceeded its deactivation threshold value. Since the engine coolant is utilized for heating the air flow which can be blown into the vehicle interior, the engine coolant (which was brought to its threshold value temperature) can sufficiently heat the air current such that warm air is blown against the vehicle's windows. Therefore, without any air drying, due to reheating the air, the windows will no longer fog up.
In another embodiment of the present invention, the function to protect deactivation against icing-up (where the compressor is automatically deactivated or activated) does not take place if the evaporator temperature is under a definable second evaporator temperature deactivation threshold value (which is higher than the first evaporator temperature deactivation threshold value) and is set, for example, to approximately 1.degree. C.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.